Hi
Here is the source I used a number of years ago to replace the Mazda bulbs in my CMP
http://classicandvintagebulbs.com/. Now they seem to only to be doing LED and Halogen. Though the home base for the company is Australia they have dealers covering US, Canada, United Kingdom and Europe.
I have one bulb left but am not sure which truck it fits. But as I said have to replace one headlight bulb this week so will see if it happens to be the one I need if so I can give you the number and which bulb base it is.
Looking at the C15TA it looks like they used the standard headlight unit from Patter 11 and 12 but hard to tell from photos.
Cheers Phil
PS Lynn, all three of my CMPs came with the big generators so they put out up to just shy of 50 AMPs on high charge then tapper down. So running the 6 volt halogen bulbs did matter. But you are very correct you don't want the headlights on for long with the engine not running above idle speed.